In the bio-medical field it is frequently necessary to manufacture plastic tubing having axial sections of different physical and/or chemical characteristics. For example, in the manufacture of suction catheters which are designed to be inserted through a patient's mouth and esophagus and into his lungs, it is desirable to make the forward or distal end of the catheter tube relatively soft to minimize the risk of damage to the patient's esophagus and lungs during insertion, and to make the rear or proximal end of the tube relatively stiff to facilitate insertion and positioning of the tube.
Plastic tubing having axial sections of different characteristics, such as catheter tubes having a relatively soft distal end and a relatively stiff proximal end, for example, is typically made by extruding a continuous length of tubing having intermittent axial sections of different characteristics (e.g., a relatively stiff axial section, followed by a relatively soft axial section, followed by a relatively stiff axial section, followed by a relatively soft axial section, etc.). Thereafter, a section of the catheter tubing having one end relatively stiff and the other end relatively soft is cut from the continuous tube.
Apparatuses for extruding plastic tubing having alternate axial lengths of different materials or characteristics are shown and described in U.S. Pat. Nos. 3,752,617 and 3,724,985.
The apparatus of U.S. Pat. No. 3,752,617 includes two or more extruders that feed different plastic materials to a mixing and extruding die. The apparatus includes a sequencing control for decreasing or interrupting the flow from one extruder and simultaneously increasing the flow from the other extruder and, after a predetermined time interval, reversing this operation to produce a tube having repeating different characteristics. The extruded tube may thereafter be cut into sections, including portions containing each characteristic. While the apparatus of U.S. Pat. No. 3,752,617 has been found to be an acceptable apparatus for extruding plastic tubing having alternate axial sections of different materials and/or characteristics, it is subject to the disadvantage of producing an undesirable amount of waste material. This waste material is produced by each extruder when the other extruder is feeding into the mixing and extruding die.
The extrusion apparatus of U.S. Pat. No. 3,724,985, and particularly the embodiment shown in FIG. 2 of that patent, was designed, in part, to eliminate the production of waste material. To that end, the apparatus of the FIG. 2 embodiment of U.S. Pat. No. 3,724,985 is equipped with accumulator-injector mechanisms, one accumulator-injector being associated with each of the two extruders. Thus, the apparatus of U.S. Pat. No. 3,724,985, as shown in the FIG. 2 embodiment thereof, has two extruders that feed different molten plastic materials to a die, there being provided an accumulator-injector in each of the conduits interconnecting the extruders with the die. The accumulator-injectors are arranged and controlled so that when one extruder is delivering molten material to the die producing a first section of tubing, a first accumulator-injector in the conduit connecting the other extruder with the die withdraws molten plastic from that conduit. Thereafter, the first accumulator-injector injects the previously-withdrawn molten material into the die, thereby eliminating waste, while the second accumulator-injector withdraws the molten material emanating from its associated extruder. The cycle is reversed periodically to produce a tube having sections of different material without requiring a valve for draining the molten material from one of the extruders while the other feeds the die.
The apparatus of U.S. Pat. No. 3,724,985 is subject to several difficulties and deficiencies. In the first place, the apparatus of U.S. Pat. No. 3,724,985 is relatively complex and expensive in that it requires accumulator-injectors along with the extruders and flow dividers and valving to properly regulate the flow of the plastic materials. Another disadvantage of the apparatus of U.S. Pat. No. 3,724,985 is that its utility is limited because of its inability to extrude relatively long lengths of tubing. This inability to extrude longer lengths of tubing arises because the plastic material to be extruded, particularly when the material is a vinyl, must remain in the extruder for extended periods of time, and degradation of the material occurs, i.e., the material burns, discolors and/or decomposes.